1. Introduction
This invention relates to electroplating nonconductors, and more particularly, to processes and compositions for electroplating the surface of a nonconductor using a stabilized preformed dispersion of a conductive polymer that functions as a base for direct electroplating when adsorbed onto a nonconducting surface. The invention is especially useful for the manufacture of electronic devices such as printed circuit boards.
2. Description of the Prior Art
Nonconducting surfaces are conventionally metallized by a sequence of steps comprising catalysis of the surface of the nonconductor to render the same catalytic to electroless metal deposition followed by contact of said catalyzed surface with a plating solution that deposits metal over the catalyzed surface in the absence of an external source of electricity. Metal plating continues for a time sufficient to form a metal deposit of the desired thickness. Following electroless metal deposition, the electroless metal deposit is optionally reinforced by electrodeposition of a metal over the electroless metal coating to obtain a desired thickness. Catalyst compositions useful for electroless metal plating are known in the art. A typical catalyst composition consists of an aqueous colloid of a noble metal such as palladium.
Electroless plating solutions are aqueous solutions containing both a dissolved metal and a reducing agent in solution. The presence of the dissolved metal and reducing agent together in solution results in plate-out of metal when the solution is in contact with a catalytic surface. However, the presence of the dissolved metal and reducing agent together in solution can also result in solution instability and indiscriminate deposition of metal on the walls of containers for such plating solutions. In addition, such plating solutions utilize a complexing agent to hold the metal dissolved in solution. Complexing agents are difficult to waste treat. The reducing agent for such solutions is typically formaldehyde. Formaldehyde is toxic and subject to environmental regulations.
For the aforesaid reasons, attempts have been made to avoid use of an electroless plating solution by the practice of a direct plating process whereby a metal may be deposited directly onto an electrically nonconducting substrate treated to render the same semiconductive. One such process is disclosed in U.S. Pat. No. 3,099,608, incorporated herein by reference. The process of this patent involves treatment of nonconducting surface with a palladium colloid which forms a semi-conductive film of colloidal palladium particles over the nonconductive surface. For reasons not fully understood, it is possible to electroplate directly over the catalyzed surface of the nonconductor from an electroplating solution though deposition occurs by propagation and growth from a conductive surface. The deposit grows epitaxially along the catalyzed surface from this interface. For this reason, metal deposition onto the substrate using this process is slow. Moreover, deposit thickness may be uneven with the thickest deposit occurring at the interface with the conductive surface and the thinnest deposit occurring at a point most remote from the interface.
U.K. Patent No. 2,123,036 13, incorporated herein by reference, is said to disclose an improvement to the above-described process. In accordance with the process of this patent, a surface is provided with metallic sites and said surface is electroplated from an electroplating solution containing an additive that is described by the patentee as inhibiting deposition of metal on metallic surfaces without inhibiting deposition on the metallic sites over the nonconducting surface. In this way, there is said to be preferential deposition over the metallic sites with a concomitant increase in the overall plating rate. In accordance with the patent, the metallic sites are preferably formed in the same manner as in the aforesaid U.S. Pat. No. 3,099,608--i.e., by immersion of the nonconducting surface in a solution of a tin-palladium colloid. The additive in the electroplating solution responsible for inhibiting deposition is described as one selected from a group of dyes, surfactants, chelating agents, brighteners and leveling agents. Many of such materials are conventional additives for electroplating solutions.
Further improvements in processes for the direct electroplating of nonconductors are disclosed in U.S. Pat. Nos. 4,895,739; 4,919,768 and 4,952,286, each incorporated herein by reference. In accordance with the processes of these patents, an electroless plating catalyst is treated with an aqueous solution of a chalcogen, such as a sulfur solution to convert the catalytic surface to a chalcogenide. By conversion of the surface to the chalcogenide, greater conductivity is achieved and faster plating rates are possible.
The processes claimed in the aforementioned patents provide a substantial improvement over the process described in the U.K. Patent. However, it has also been found that treatment of an adsorbed catalytic metal over a nonconductor with a solution of a chalcogenide, especially a sulfide solution, results in formation of a chalcogenide on all metal surfaces in contact with the solution of the chalcogen. Therefore, if the process is used in the manufacture of printed circuit boards, the copper cladding or conductors of the printed circuit board base material are converted to the chalcogenide together with the catalytic metal. If the chalcogenide of the copper is not removed prior to plating, it can reduce the bond strength between the copper and a subsequently deposited metal over the copper.
A further advance in the direct plating of the surfaces of nonconductors is disclosed in published European Application No. 0 520 1915, incorporated herein by reference. In accordance with the invention described therein, a stable colloidal solution of a preformed catalytic chalcogenide is prepared and a surface is then contacted with the colloidal composition whereby the colloidal chalcogenide adsorbs on said surface. Thereafter, the nonconductor may be electroplated following procedures disclosed in the aforesaid U.S. Pat. Nos. 4,895,739; 4,919,768 and 4,952,286.
An alternative direct plate process is disclosed in PCT published application No. 89/00204, incorporated herein by reference. In accordance with the process of said published application, the surface of a substrate is pretreated with a solution having an oxidizing capability, removed from said solution and rinsed, introduced into a solution containing a monomer such as a pyrrole, furane, thiophene or its derivatives, which in a polymeric or copolymeric form is electrically conductive; the surface is then transferred into an acidic solution whereby an electrically conductive polymer layer, such as polymerized or copolymerized pyrrole, furane, thiophene or derivative is formed, residual solution is removed by rinsing; and the coating formed over the substrate is then semiconductive and capable of direct electroplating. In accordance with this process, the oxidative pretreatment solution contains salts of permanganate, manganate, periodate and/or cerium IV. The monomer is present in a suitable solvent in an amount of from 5 to 35 % by weight. The substrate is immersed in the monomer solution for a time ranging from about several seconds to 20 minutes. Room temperature treatment is satisfactory. The solution used to activate the monomer may contain an oxidative substance such as alkali metal persulfate, alkali metal peroxydisulfate, hydrogen peroxide, an iron salt such as ferric chloride, alkali metal periodates or similar compounds in acidic solution. A solution containing an active oxidizing agent in an amount of from 25 to 75 grams per liter of solution is usually considered satisfactory. Treatment can be at room temperature with immersion times of from 1 to 10 minutes and treatment is complete when a dark brown or black coloration is formed on the surface of the substrate. Difficulties encountered with this procedure include incomplete coverage of glass fibers and or epoxy or FR4 epoxide/glass printed circuit board substrate which can lead to discontinuities in the coating and the use of volatile organic compounds in the process sequence which may be toxic.
A modification to the above process is disclosed by Gottsfeld at al, J. Electrochem. Soc., Vol. 139, No. 1, January 1992, pp. 14-15. In this process, a substrate to be plated is immersed directly into a solution of the oxidant and the monomer is then added to the solution to cause in situ formation of polymer in the presence of the substrate. Disadvantages to this process include the formation of polymer on all surfaces in contact with the solution--i.e., the container walls as well as the substrate, a limited life of the treatment solution and possible monomer toxicity.
In U.S. Pat. No. 5,415,762 incorporated herein by reference, an additional method for direct electroplating of nonconducting surfaces is disclosed. This method uses an aqueous or semi-aqueous suspension of a conductive polymer. The suspension may be in the form of a colloid or emulsion dependent upon the physical form of the conductive polymer. Using this suspension, a circuit board would be fabricated by a process where the first step comprises preparation of the circuit board substrate for plating. This includes formation of throughholes, desmearing of the hole walls, conditioning of the hole walls to improve adhesion and etching of the copper cladding to clean the same. Thereafter, the substrate is contacted with the suspension of the conductive polymer. The suspension is desirably formulated to be attracted to the conditioned dielectric surfaces. Thereafter, the substrate is electroplated in conventional manner. The patent further discloses that any conductive polymer suitable for suspension in aqueous media may be used with preferred conductive polymers including polypyrrole, polyaniline and polythiophene or derivatives thereof.
The suspension of a conductive polymer disclosed in U.S. Pat. No. 5,415,762 functions as described in said patent. However, it has been found that in storage, the particles may aggregate and precipitate. This limits the commercial suitability of the suspension as it is difficult to package the same and store the suspension for a prolonged period of time prior to use. It is known in the art that suspensions of organic particles may be stabilized to prevent aggregation, but it has been found in practice that these stabilizers tend to reduce the conductivity of a coating of organic conductive polymer particles adsorbed onto a substrate making it difficult to obtain complete coverage during electroplating.